Good question - and --platform agents does exist. It's just not a universal installer; it's one target among many. It drops the skill in ~/.agents/skills/ and wires up AGENTS.md, which is the cross-framework "Agent Skills" convention (npx skills and spec-compliant tools read it). If your agent follows that spec, that's all you need.

The per-agent commands exist because most agents don't follow them. They differ in ways one command can't paper over:

• Where they look — Claude reads ~/.claude/skills/, Cursor uses .cursor/rules, Amp ~/.config/agents/skills/ (not ~/.agents/skills), Gemini ~/.gemini/skills/. No two agree.
• What format — Cursor wants rules, not a skill. Aider/Devin wants one monolithic file; Claude/Codex uses a lean skill plus a references/ sidecar. Windows ships a PowerShell script, not bash.
• How it's registered — Gemini installs a settings hook, OpenCode a plugin, Claude appends a line to CLAUDE.md. A generic copy can't do those.

So --platform agents covers anything speaking the AGENTS.md convention; the rest exist because those tools each picked their own directory + format + wiring, and installing the wrong shape just silently doesn't load.